It appears hugo-extended is no longer the default build. I'm on Linux / Ubuntu, so would appreciate if someone check that it is also no longer the default via `brew install` (I assumed the change was consistently throughout, based on github changes).
While Hugo does not care if the section is named "post" or "posts",
this commit normalizes the whole Hugo documentation to use "posts"
when used as an example for a section name to prevent user confusion.
Fixes https://github.com/gohugoio/hugoDocs/issues/720.
Also highlight the use of `site.Params.mainSections`.